In a work vehicle that does not include a cab suspension system, the ride quality and operator comfort of the work vehicle is adversely affected by vibrations or movement transmitted from the frame or chassis of the vehicle to the operator's cab. As the work vehicle travels across a surface, movement of the chassis induces the operator's cab to pitch, roll and bounce. Movement of the cab can be particularly severe in agricultural and construction equipment vehicles (e.g., tractors, combines, backhoes, cranes, dozers, trenchers, skid-steer loaders, etc.) because such vehicles typically operate on off-road surfaces or fields having a high level of bumpiness.
Operator comfort may also be adversely affected by the operation of various systems on a work vehicle. In particular, operation of various work vehicle systems can cause forces to be applied to the chassis of the vehicle which, in turn, are transmitted to the cab. Examples of these forces include the following: draft forces exerted on the hitch of an agricultural tractor by an implement (e.g., a plow) which can cause the cab to pitch; normal forces applied to a work vehicle as the vehicle turns in response to a steering device which can cause the cab to roll; clutch forces generated when a work vehicle clutch (e.g., a main drive clutch; four-wheel drive clutch) is engaged or disengaged which can cause the cab to pitch; gear shift forces generated when a transmission of a work vehicle is shifted which can cause the cab to pitch; braking forces generated as brakes of a work vehicle are operated which can cause the cab to pitch; acceleration forces generated when a speed actuator changes the speed of a work vehicle which can cause the cab to pitch; etc.
The movement of the cab caused by surface bumps and the operation of vehicle systems cause both qualitative and quantitative problems. An operator of such a vehicle experiences increased levels of discomfort and fatigue caused by the vibrations. Productivity is decreased when an operator is forced to rest or shorten the work day, or is unable to efficiently control the work vehicle. The operator is also less likely to be satisfied with a work vehicle which provides poor ride quality. Under certain conditions, the frequency and magnitude of cab movement may force the operator to decrease driving speed, further decreasing productivity.
To improve ride quality and operator comfort, work vehicles have been equipped with passive, semi-active or active suspension systems to isolate the operator from vibrations caused by surface bumps. Such systems include vibration isolators mounted between the chassis and cab or seat. Passive systems use passive vibration isolators (e.g., rubber isolators, springs with friction or viscous dampers) to damp vibrations with different isolators used to damp different frequencies. Rubber isolators may be used, for example, to damp high frequency vibrations and air bags used to damp low frequency vibrations. However, performance of passive systems is limited due to design compromises needed to achieve good control at resonance frequencies and good isolation at high frequencies.
Semi-active systems achieve control and isolation between the chassis and the cab by controlling a damper to selectively remove energy from the system in response to movement of the cab sensed by sensors. Active systems use sensors to sense cab movement and a controller to generate control signals for an actuator which applies a force to the cab to cancel vibrations transmitted to the cab by the chassis. The power needed to apply the force is supplied by an external source (e.g., hydraulic pump).
As the above paragraphs imply, it is desirable that a suspension system attenuate both low and high frequency vibrations between the chassis and cab. Attenuation of high frequency vibrations can decrease acoustic noise in the cab, decrease fatigue and decrease vibration-induced mechanical faults. Attenuation of low frequency (e.g., less than 20 Hz) vibrations can decrease operator fatigue and improve vehicle operability. The attenuation of low frequency vibrations is particularly important because the resonant frequencies of the human body are typically below 20 Hz. For example, the human abdomen resonates at frequencies between 4-8 Hz, the head and eyes resonate at frequencies around 10 Hz, and the torso at 1-2 Hz. The actual frequency may vary with the particular individual.
With respect to active suspension systems, various active suspension systems for a work vehicle are known. One active suspension system for a work vehicle includes a hydraulic actuator mounted at a single point between the rear of the cab and the vehicle frame. The front of the cab is pivotally mounted to the frame. The actuator is controlled to move the cab relative to the frame in response to sensed acceleration signals. The system includes a single air bag used to level the cab. Another active suspension system for a work vehicle includes one active vibration isolator mounted between the vehicle chassis and the rear of the cab, and two active isolators mounted between the chassis and the front of the cab. Each isolator includes a hydraulic actuator mounted between the chassis and the cab, and an air bag to support the weight of the cab. The actuator is controlled to move the cab relative to the chassis in response to sensed acceleration signals. Each isolator is individually controlled by an electronic controller replicated for each isolator.
Although work vehicles having active cab suspension systems provide improved ride quality and increased operator comfort, the active cab suspension systems neither are designed for nor are capable of eliminating all undesirable vibration and movement experienced within the cabs or by other components of the work vehicles. As a work vehicle proceeds down a path, the work vehicle (e.g., the chassis of the work vehicle) continually experiences vibration due to variation in the level of the terrain being traversed. Such vibration may be of sufficient magnitude to cause damage to the work vehicle, in several ways.
If the vibration is significant but does not overly strain the active cab suspension system, the active cab suspension system will shield the operator of the work vehicle from much or all of the vibration, and the operator will likely remain unaware of the magnitude of the vibration. In this case, although the operator will not physically experience all of the vibration, the vibration may still be significant enough to cause damage to the work vehicle, particularly to components of the work vehicle other than the cab such as the chassis. Additionally, the vibration may be significant enough to cause damage to implements being towed by (or otherwise attached to) the work vehicle (the work vehicle may be defined as including such implements). Thus, it is possible for the work vehicle to become damaged due to excessive vibration because the active cab suspension system precludes operator awareness of the vibration and, consequently, the operator continues to operate the work vehicle without taking affirmative action to reduce the excessive vibration, such as slowing down the vehicle's travel speed.
Further, if the vibration becomes even greater, the active cab suspension system may be strained beyond its operational capabilities. In particular, excessive vibration may cause one or more of the active vibration isolators to exceed their predetermined limits of physical movement and may even cause one or more of the hydraulic pistons within the respective active vibration isolators (or other components of the active cab suspension system) to reach their mechanical stops. Under these conditions, damage may occur to both the active cab suspension system and to other components of the work vehicle (including the cab). While the operator of the work vehicle will become aware of the excessive vibration (and presumably take action to reduce the vibration) if the hydraulic pistons reach their mechanical stops, significant damage may already have occurred by that time. That is, the vibration may be placing excessive strain upon (or otherwise damaging) the active cab suspension system and other components of the work vehicle before the hydraulic pistons reach their mechanical stops, and before the operator can take action to prevent the excessive vibration.
Additionally, in certain cases a work vehicle may experience a particular type of vibration from which its active cab suspension system is incapable of isolating the cab. While such vibration is therefore physically experienced by the operator of the work vehicle (since the active cab suspension system is incapable of preventing transmission of the vibration to the cab), the operator may remain unaware or unconcerned with the vibration even though it may be undesirable for the operator to be exposed to such vibration for longer than a given period of time. Suggested maximum times of exposure of a person to vibrations of particular frequencies and magnitudes are provided in ISO 2631 set forth by the International Standards Organization.
Accordingly, it would be advantageous to develop a system for facilitating the reduction of excessive vibration in a work vehicle having an active cab suspension system. It would be advantageous if such a system in particular facilitated the reduction of excessive vibration in three cases, namely: (1) when such vibration is capable of causing damage to components of the work vehicle (including any towed implements) even though the vibration is not so significant as to exceed the capabilities of the active cab suspension system for preventing that vibration from being transmitted to the cab of the work vehicle; (2) when such vibration is so significant as to place excessive strain upon the active cab suspension system, and is capable of causing damage both to the active cab suspension system and other components of the work vehicle (including the cab); and (3) when such vibration is of a frequency and/or magnitude such that the active cab suspension system is incapable of preventing transmission of the vibration to the cab and it is nonetheless desirable to limit the exposure of the operator of the work vehicle to that vibration.